1. Field of the Invention
The present invention relates to a photo-fixed color image forming developer used in electrophotography, electrostatic recording, and magnetic recording, and to a color image forming method and a color image forming device using the same.
2. Description of the Prior Art
Generally, in electrophotography, which is widely employed in copiers and printers, a photoconductive insulator surface in a photoreceptor drum is electrified with uniform static charges, which may be positive or negative. A charged photoconductive insulator surface is then irradiated with light to form a latent image by means of partially erasing the static charges on the surface. For example, a latent image produced as a result of image information can be formed on a charged photoconductive insulator surface by irradiating the surface with laser beams in response to image information to erase the surface charges hit by the beams. Then, a toner in a developer assuming the form of fine particles is deposited onto the latent image charged on the surface, to thereby visualize the image. The toner image is formed on the photoconductive insulator surface. The resultant toner image is then electrostatically transferred onto a recording medium, such as paper.
The transferred toner image is fixed on the recording medium, where the toner is molten when transferred onto the medium, and then solidified/fixed on the surface. The toner is rendered molten by means of elevated pressure and/or temperature, or by the aid of light. Flash fusing has been attracting attention, because it is free of problems caused by elevated pressure or temperature.
Flash fusing generally has the following advantages: (i) deterioration of image resolution (repeatability) is lowered, because fixing the toner does not require pressurization of the toner, and therefore the toner does not have to come into contact with and receive pressure from a fuser roller; (ii) printing can be started as soon as the power source is switched on, unlike the case of conventional technique, which involves a time delay before a heat source (a fuser roller or the like) is preheated to a desired temperature level; (iii) provision of a high-temperature heat source is not necessary, and therefore the device does not undergo a substantial temperature rise; and (iv) there is avoided the situation where paper is ignited by heat from a heat source when jammed in the fuser device while the system is down.
In spite of these advantages, flash fusing involves a problem of insufficient fixation capacity when a color toner is used, because a color toner, which has lower light-absorbing capacity than a black toner, may fail to absorb sufficient light to convert its energy into heat, resulting in insufficient melting in the fixation step. Various attempts have been made to improve fixation capacity by means of incorporating into a toner an infrared absorber serving as a light absorber, as disclosed by a number of patent documents; e.g., Japanese Patent Laid-Open Publication Nos. 60-63545, 60-63546, 60-57858, 60-57857, 58-102248, 58-102247, 60-131544, 60-133460, 61-132959, 2000-147824, Hei 7-191492, 2000-155439, Hei 6-348056, Hei 10-39535, 2000-35689, Hei 11-38666, Hei 11-125930, Hei 11-125928, Hei 11-125929, and Hei 11-65167. These patent documents disclose techniques for incorporating into a toner an agent capable of absorbing light in the infrared region, serving as an infrared absorber, to improve flash fusing capacity, thereby solving problems resulting from insufficient melting capacity. In addition to use of the absorber, increasing emission capacity of a photo-fixer is another technique for improving fixation capacity of a toner.
Although exhibiting improved fixation capacity, a toner containing an excessive quantity of infrared absorber, generates excessive heat as a result of absorbing an excessive quantity of light, thereby causing printing defects referred to as “voids” that are left by the toner, moisture in the medium, or the like. Therefore, infrared absorber content of toner must be determined in view of its color, in order to simultaneously attain sufficient resistance to void formation and sufficient fixation capacity.
However, some of the color image forming developers proposed by the above patent documents comprise cyan, magenta, and yellow toners that contain an infrared absorber, and encounter difficulty in simultaneously attaining sufficient resistance to void formation and sufficient fixation capacity during the fixation step.
As a result of conducting extensive studies, the present inventors have found that the above problems result from a cyan pigment in a cyan toner having higher capacity of absorbing visible light in a wavelength range of 600 to 800 nm as compared with magenta and yellow toners. When the same infrared absorber in incorporated into the respective toners at the same content, total quantity of light absorbed varies according to visible ray absorbing capacity. This makes it difficult to simultaneously attain sufficient fixation capacity and sufficient resistance to void formation.
When emission intensity during the fixation step is increased in accordance with flash fusing capacity of magenta and yellow toners, although these toners exhibit good fixation capacity, resistance of the cyan toner to void formation may deteriorate, because the cyan toner can absorb excessive visible light to thereby form voids. Meanwhile, when emission intensity during the fixation step is lowered to an extent to avoid formation of voids by the cyan toner, although the cyan toner exhibits sufficient fixation capacity, sufficient fixation capacity of the magenta and yellow toners cannot be secured, because the magenta and yellow toners, which have lower visible light absorbing capacity than a cyan toner, cannot absorb sufficient light to melt sufficiently. Therefore, sufficient fixation capacity and sufficient resistance to void formation cannot be attained simultaneously when emission intensity is set in accordance with these properties for a cyan toner or magenta/yellow toners, the former having higher visible light absorption capacity than the latter.